1. Field of the Invention
The present invention relates to an automatic transmission for a vehicle and, more particularly to an automatic transmission including planetary gear units each incorporating a centrifugal clutch.
2. Description of the Related Art
A known automatic transmission disclosed in U.S. Pat. No. 5,514,043 includes four planetary gear units of helical gears each incorporating a centrifugal clutch that is engaged by flyweights that are moved radially by centrifugal force and is disengaged by thrust exerted on the helical gears of the planetary gear unit. The four planetary gear units are connected in series. Each planetary gear unit has a ring gear mounted on an input shaft and serving as an input member, and a planet carrier mounted on an output shaft and serving as an output member. The centrifugal clutch is interposed between the ring gear and the planet carrier. The planetary gear unit is a two-speed transmission that reduces the rotating speed of the input shaft and lowers the rotating speed of the output shaft when the centrifugal clutch is disengaged, and rotates the input shaft and the output shaft at the same rotating speed for direct drive when the centrifugal clutch is engaged.
The four planetary gear units are connected in series to set the automatic transmission for a first speed by disengaging all the four centrifugal clutches, for a second speed by engaging one of the four centrifugal clutches, for a third speed by engaging two of the four centrifugal clutches, for a fourth speed by engaging three of the four centrifugal clutches and for a fifth speed by engaging all the four centrifugal clutches. When the automatic transmission is set for fifth speed by engaging all the four centrifugal clutches, the output shaft of the automatic transmission rotates at a rotating speed equal to that of the input shaft of the same to serve as a transmission of a gear ratio of 1 to 1.
When the automatic transmission is set for the highest speed, i.e., the fifth speed, the input shaft is connected directly to the output shaft for direct drive in which the rotating speed of the output shaft is equal to that of the input shaft. Therefore, this automatic transmission cannot function as an overdrive. Thus, the planetary gear units nearer to the output end of the automatic transmission must have greater torque capacities, and the component gears, clutches and bearings of those planetary gear units must bear considerably large load and hence must be formed in large units requiring large spaces for installation to secure necessary strength.
The present invention has been made in view of those problems and it is therefore an object of the present invention to provide a small multispeed automatic transmission for a vehicle employing planetary gear units of helical gears each including a centrifugal clutch that is engaged by the action of flyweights operated by centrifugal force and is disengaged by thrust exerted on the helical gears, and capable of functioning as an overdrive.
According to a first aspect of the present invention, an automatic transmission for a vehicle comprises: an input shaft connected to the output shaft of an engine; a first planetary gear unit including a helical sun gear, helical planet pinions supported for rotation on a planet carrier, and a helical ring gear, the sun gear (or the ring gear) being connected to the input shaft, the ring gear (or the sun gear) being connected through a one-way clutch to a fixed member such that the same can be restrained from rotation relative to the fixed member, the planet carrier being connected to a first output shaft; a first clutch interposed between the input shaft and the first output shaft, and capable of being engaged by the action of flyweights operated by centrifugal force and of being disengaged by thrust exerted on the helical gears; a second planetary gear unit including a sun gear, planet pinions supported for rotation on a planet carrier, and a ring gear, the planet carrier being connected to the first output shaft, the sun gear (or the ring gear) being connected to a second output shaft, the ring gear (or the sun gear) being connected through a one-way clutch to a fixed member such that the same can be restrained from rotation relative to the fixed member; a second clutch interposed between the first and the second output shaft, and capable of being controlled by a control means so as to engage the first and the second output shaft or disengage the first and the second output shaft from each other; a third planetary gear unit including a helical sun gear, a helical planet pinions supported for rotation on a planet carrier, and a ring gear, the sun gear (or the ring gear) being connected to the second output shaft, the ring gear (or the sun gear) being connected through a one-way clutch to a fixed member such that the same can be restrained from rotation relative to the fixed member; and a third clutch interposed between the second and the third output shaft, and capable of being engaged by the action of flyweights operated by centrifugal force and of being disengaged by thrust exerted on the helical gears.
Since the second planetary gear unit that operates as a two-speed gear unit capable of being set for either of a direct-drive speed and a step-up speed is interposed between the first planetary gear unit that operates as a two-speed gear unit capable of being set for either of a direct-drive speed and a reduction speed and the third planetary gear, the automatic transmission is capable of providing eight speeds including an overdrive speed.
Accordingly, the torque capacities of the component members on the lower side of the power transmission train may be small, load on the gears, clutches and bearings on the lower side of the power transmission train is reduced, so that the automatic transmission can be formed in a small, lightweight unit that can be installed in a small space.
Preferably, the control means engages the second clutch of the automatic transmission while the vehicle is stopped.
When the second clutch is engaged with the vehicle stopped on an uphill road, the transmission of a torque tending to turn the driving wheels of the vehicle in the reverse direction is prevented by the one-way clutch of the second planetary gear unit. Consequently, the backward movement of the vehicle can be prevented without requiring creep control and fuel consumption can be reduced.
Preferably, the automatic transmission further comprises a reversing mechanism mounted on the third output shaft.
When the reversing mechanism is mounted on the third output shaft disposed on the downstream side that may have a small torque capacity when the automatic transmission is set for an overdrive speed, load on a clutch, gears and bearings included in the reversing mechanism is reduced and the reversing mechanism can be formed in a small, lightweight unit.